Language Disturbances in Adulthood: New Advances from the Neurolinguistics Perspective

Language is the “most human” of all cognitive abilities. Hence, understanding how our brains process language has intrigued neuroscientists since the first description of an aphasic patient by Paul Broca at the end of the eighteenth century. Much of our knowledge about language processing derives from the observation of signs and symptoms caused by brain injury, but normal language is also a topic of investigation among researchers, from Psycholinguistics to Computational Sciences. However, the exact nature of linguistic processing is yet to be fully understood. Oral language comprehension seems to be based on the auditory discrimination of word subunits, which vary depending on the language considered: stress units (English), syllables (French), mora (Japanese), and so on. These subunits must then be grouped and a selection mechanism has to discriminate among similar words to avoid ambiguities. Grammatical rules contribute to organize words (in their morphosyntactic aspects), and, finally, context and prosody give their contribution so that the listener is able to understand the content of discourse, as in a conversation. Language production relies on the phylogenetic development of a specific supra-laryngeal articulation mechanism under neocortical control, and also of a social competence now described as Theory of Mind (ToM). Language production starts with the mental generation of a message to be conveyed; this message has to be translated into grammatically encoded words. Grammatical encoding includes lemma selection, morphosyntactic composition, morphophonological and phonetic encoding, in a complex process at the end of which the message (words) is uttered by the speaker.

Aphasia can be defined as an acquired deficit in language processing due to dysfunction in specific areas of the brain. The classic aphasias (Broca's, Wernicke's and Global) are representative of impairment in the very epicenters of language areas, thus inducing signs and symptoms that affect all levels of linguistic processing: phonological, syntactical, lexical, and semantic. However, there is a group of aphasias in which the lesion site spares the classic language areas, thus inducing more subtle signs and symptoms, and leading to a combination of linguistic symptoms mixed with those arising from other cognitive functions also affected by the same lesion. That is the case with the so called “transcortical aphasias” and “conduction aphasia”. Transcortical aphasias are classified as motor, sensory, and mixed. In transcortical motor aphasia (TCMA), the medial and the dorsolateral frontal lobes are compromised, interrupting the connections between these two systems and language areas; TCMA patients have impairment performance on tasks demanding speech generation (diminished fluency, impairment in narrative, and in the production of grammatically complex sentences), and a reduction in the motivation to engage in communication. In transcortical sensory aphasia (TCSA) there is a disorder in semantic processing, manifesting as comprehension and naming impairment, which indicates a disconnection between posterior language areas and semantic memory. In mixed transcortical aphasia (MTA) there is a combination of signs and symptoms of both TCMA and TCSA in varying degrees. Conduction aphasia, in which the core symptom is repetition deficit, represents a primary failure of the phonological loop.

The role of right hemisphere (RH) in language and communication has been a subject of debate over the last decades. Studies derived from RH lesions and functional neuroimaging in the past years have, nevertheless, shed some light on the multiple roles of the RH in linguistic processing and several aspects of communication in a broader sense. It is now recognized that the RH plays an important role in decoding “non-literal” language, such as emotional prosody, metaphors and figurative language interpretation. However, the RH also plays an important role in discourse elaboration (macrostructure) and pragmatics, as well as in “grasping” the implicit meaning in cases of irony, motivations, and emotional states that are often embedded in daily communication.

Cognitive functions are intermingled and are known to depend upon each other for optimal efficiency. Linguistic performance depends on a series of cognitive domains known as “language supportive functions”. Among these are included attention, working memory, executive functions and visuospatial abilities. Attention plays a major role in several steps of linguistic processing, ranging from lexical selection to the comprehension of complex material (such as sentences and texts) and also writing. The phonological loop (the verbal component of working memory), which accounts for the transient manipulation of verbal material, is critical for adequate syntactic comprehension and vocabulary acquisition. Working memory deficits can interfere with the ability to comprehend texts, as well as discourse and conversation. Executive dysfunction interferes with the subject's ability to access semantic and lexical representations. Executive functions are also involved in sentence and discourse production, as they are necessary for planning how to express an idea. Visuospatial perception abilities may interfere with naming tasks, when subjects are unable to name an object because they cannot recognize it. Finally, it has been demonstrated that aphasia itself may hinder the subject's performance in several cognitive tasks, such as color-picture matching, picture ordering, calculation and drawing from memory. Moreover, the interface between language and other cognitive functions becomes evident in neuropsychological testing, as most tests require input and reasoning for verbally presented material (starting with test instructions), and frequently the output must also be verbal (oral or written), even when non-language functions are being tested.

Language disturbances are frequently found in several forms of dementia and may greatly compromise patients' daily performance in cognitive and social tasks, as well as impair their quality of life. In this text the linguistic-cognitive alterations of aging, emphasizing language, will be presented and alterations in ‘ mild cognitive impairment’ (MCI) shall be briefly highlighted. Subsequently, the language characteristics of different dementias will be addressed: Alzheimer's disease (AD), Vascular Dementia (VD), Frontotemporal Dementia (FTD) and Primary Progressive Aphasia (PPA). The hallmark of language impairment in AD is related to lexico-semantic aspects, while in VD word recognition, naming and repetition are more impaired. Impairment of language in VD is in great part a consequence of a disexecutive syndrome (including working memory dysfunction). In FTD, discourse organization and pragmatic alterations predominate. PPA can be subdivided in several clinical subtypes including agrammatic, semantic and logopenic deficits. Prominent semantic deficits appear in the so-called Semantic PPA (SPPA); in nonfluent PPA (NFPPA), the language production is reduced and effortful with marked agrammatism and difficulties in comprehension of complex grammatical structures. Logopenic PPA (LPPA) is characterized by deficits in lexical recuperation and repetition of sentences, with preserved word comprehension, and absence of agrammatism.

The role of subcortical structures in language processing is a topic of intense debate in the literature. Language alterations resulting from subcortical damage, such as the basal ganglia and thalamus do not produce classical aphasia syndromes as those encountered in cortical lesions. Basal ganglia lesions (caudate nucleus, putamen, globus pallidus) produce a complex and varied symptomatology regarding language manifestations, being the more consistent an alteration in lexical selection processes. More recent theories implicate the striatum in sequential and computational aspects of language processing, combinatorial rule applications and procedural learning (which are related to morphology and syntax), as well as stuttering. Thalamic aphasias present a more homogeneous pattern, where anomia and semantic paraphasias predominate, pointing to the role of the thalamus in mechanisms of cortical engagement and semantic verification. There has been increasing evidence of the participation of the cerebellum in language processing. Among the language deficits found after cerebellar lesions are delay in language acquisition, deficits in speech initiation, and mutism. Other previously reported language alterations include deficits in speech production, such as short responses, difficulties in initiating a conversation, long latencies for answers, and word finding difficulties.

Functional neuroimaging methods allow the study of healthy subjects and patients while they perform selected tasks, in a relatively non-invasive manner. PET and fMRI studies have greatly contributed to our understanding of language processing in the past decades. Regarding language comprehension, studies indicate that the posterior region of the left superior temporal gyrus, or Wernicke's area, is divided into several functional subareas which are involved in phonemic perception, access to lexical representations, and retrieval of words from the semantic store; the inferior temporal cortex, the medial and lateral portions of the temporal lobe, the angular gyrus and frontal association cortex in the left hemisphere (LH) are related to semantic processing, while the left frontal inferior cortex is involved in syntactic tasks. Comprehension of discourse and texts depends on the left frontal lobe, although the macrostructural and connotational aspects of discourse interpretation demand right hemisphere (RH) activation. Other aspects of language processing that have been extensively studied using functional neuroimaging methods are bilingualism, the effects of literacy in the organization of neural networks, and the impact of speech therapy in the recovery of linguistic abilities in patients with cerebral lesions or degenerative diseases. However, one of the challenges faced by researchers in this field is how to create activation experiments that mimic the natural use of language in a more ecological way.